Pattern photic stimulator



NOV- 18, 1958 c. M. HARDEN ETAL 2,860,627

PATTERN PHo'Tc STIMULATOR 4 Sheets-Sheet l Filed March 26, 1953 Nov. 18,1958 Filed March 26. 1955 C. M. HARDEN ETAL PATTERN PHOTIC STIMULATOR 4sheets-sheet 42 ELECTRO-ENCE- PHALOGRAPH RECORDER ANALYZER INVENTORSCHARLES M. HARDEN CURTIS MARSHALL Nov. 18, 1958 c. M. HADEN ETAL PATTERNPHOTIC STIMULATOR 4 Sheets-Sheet 3 Filed Hatch 26, 1953 INVENTORSCHARLES M. HARDEN CURTIS MARSHALL Nov. 1s, 195s c. M. HARDEN ET AL2,860,627

l PATTERN PHoTIc STIMULATOR A 4 Sheets-Sheet 4 Filed March 2.6. 1953INVENTORS CHARLES M. HARDEN CURTIS MARSHALL nited States Patent lPATTERN PHOTIC STIMULATOR Charles M; Harden, Natick, Mass., and CurtisMarshall, Baltimore, Md.

Application March 26, 1953, Serial No. 344,853

12 Claims. (ci. 12s-2.1)

This invention relates generally to photo-stimulation and moreparticularly to methods and means for effecting photo-stimulation withrelatively low intensity variable light patterns.

Photo-stimulation may be considered as that branch of neurologicaldiagnosis and analysis or treatment which involves presenting to thevisual sense of a subject a non-constant source of illumination andobserving the inuence of the change in such illumination upon theneurological activity of the subject. In general, neurological activitymay best be observed and indicated by means of electroencephalographic(EEG) apparatus which is well known in the art.

The EEG response to intermittent photic stimulation is well known andhas been used to a limited extent heretofore for the treatment ofpatients having certain neurological abnormalities and for obtainingdata related to the activity of the human brain and visual apparatus ingeneral. These prior arrangements have suffered from certainshortcomings including a low sensitivity of overall response. Inattempting to produce an adequate response by increasing the intensityof the light source, the subject rapidly became fatigued and thepossibility of injury to the retina arose before signicant improvementin the EEG response was obtained. In addition, such intense lightsources were either unduly cumbersome to work with or involved the useof large transient electromagnetic fields, causing interference with thedelicate potentials measured at the surface of the scalp which wasdiicult to eliminate.

Certain other attempts to obtain EEG response to low level intermittentor undulatory light sources have been satisfactory for certain studiesbut have been incapable of presenting spatial modulation to the eye.

The foregoing disadvantages of prior art arrangements and methods havebeen largely overcome by the methods and apparatus of the presentinvention which, in addition to providing improved response from a lowintensity source` of light, permits quantitative studies to be conductedbetween the characteristics of the light source and the brain activityinfluenced thereby.

Accordingly it is an object of this invention to provide new andimproved methods and apparatus for photostimulation.

A further object of this invention is to provide arrangements forphoto-stimulation in which a variable light pattern is employed.

Another object is to provide method and means which are .readily appliedto experimental procedures and in which controlled conditions may beintroduced as required during the course of the diagnosis.

A further object is to provide photo-stimulation arrangements Whichproduce improved results without danger of injury-to the subject andwhich may be carried on for extended periods of time without unduefatigue.

A further object is to provide apparatus which will present to thesubject a light pattern of variable size or shape which has an intensitywhich is not offensive to v 2,860,627 Patented Nov. 1s, 1958 2 thesubject and which may be spatially modulated at will to product the mostdesired results.

A further object of the invention is to provide photostimulation methodsand apparatus in which the EEG response may be utilized to' control ormodulate the source of the visual stimulation.

A still further object is to provide improved arrangements for comparingnormal optical unstimulated and controlled photo-stimulated EEGresponses.

These and other objects of the present invention are obtained by meansof the present preferred embodiment of apparatus and employment oftechniques which are more fully set forth in the detailed descriptionand the accompanying drawings. Essentially the invention requires avisual pattern source which can be spatially and intensity modulatedover wide ranges, such as the cathode ray oscilloscope tub'e. Bygenerating suitable light patterns ori the fluorescent screen of thetube which are observed by the ,subject being analyzed, the EEG responseinfluenced thereby may be readily sensed by means of conventional EEGequipment. These EEG responses may be recorded and analyzed for variousfrequency c'omponents to determine correlation between the visualpattern modulation and the EEG response in a manner set forth in anarticle entitled Use of Rhythmically Varying Patterns for PhoticStimulation published in The EEG Journal for August 1952. By means ofsuitable feedback networks the EEG potentials or frequency componentsthereof may be employed to synchronize or modulate the visual pattern.

Further advantages and equivalent modifications of the method andapparatus claimed herein will become apparent from the followingdetailed description taken in conjunction with the accompanying drawingswherein:

Fig. l is a diagrammatic view of the preferred embodiment of the systemof the present invention as arranged for practicing the method of theinvention;

Fig. 2 is a block diagram of oscilloscope circuits as employed in Fig.l;

Fig. 3 is a view of a preferred luminous pattern on the fluorescentscreen of the oscilloscope of Fig. l with representations of thehorizontal and vertical deecting forces applied to the cathode-ray;

Figs. 4 and 5 are views representative of modified luminous patternswhich may be used;

Fig. 6 is a graph showing a composite record of EEG potentials and theanalyzed frequency components thereof for a subject who was staring at amaze;

Fig. 7 is a graph showing a composite record similar to Fig. 6 for thesame subject who was observing a varying light pattern in accordancewith the invention;

Fig. 8 is a partial diagram of a modified connection of a portion of theapparatus of Fig. l; and

Fig. 9 is a schematic diagram of a differential attenuator.

Referring now to Fig.A l the system of the present invention will bedescribed together with the novel methods of the present teaching. Thesystem includes a cathoderay tube 11 having a fluorescent screen 12which provides localized visible light energy upon energization thereofby a focused electron beam of predetermined energy level. In accordancewith the present invention a desired light pattern is produced on thescreen 12 by suitably deflecting the electron beam and controlling theintensity thereof. The control of the electron beam may be effected bycircuits 13 which are of a type generally well known in the oscilloscopeart and which will be described more fully hereinafter. Operation of thetube 11 to energize the screen 12 in a desired manner from the circuits13 is achieved by an energy and signal cable 14 providing the necessaryconnections therebetween.

As will be further described, a desired light pattern is produced on thescreen 12 by the generation and utilization of waveforms of current orvoltage which vary with time. These waveforms will, in general, besynthesized from more basic waveforms which are of two types. First, acarrier or super-visible-rate Waveform will generate a basic shape lightpattern such as, for example, a circle. Second, a quasi-visible-ratewaveform will, by modulating the carrier or otherwise, vary the size,shape, position or other characteristic of the light pattern. It will beunderstood that this technique is but a means to an end and any othermeans for producing light patterns, of which those herein disclosed areexemplary, are equally satisfactory for the purposes of this invention.The light pattern produced by the rst and second above describedwaveforms is such that the basic shape generated by thesuper-visible-rate waveform appears as a continuous image in which thehuman eye is unable to discern any appreciable motion. The modulation ofthis basic continuous image produced by the quasi-visible-rate waveformis carried on at speeds including those at which the human eye is ableto perceive the motion so produced. An important feature of theapparatus of the present invention and a step in the method thereof isthe adjustment of the recurrence frequence or the speed of transition ofthe quasi Visible rate waveform. To achieve this lresult a sweepfrequency adjustment control 15 is provided.

The oscilloscope circuits are provided with synchronizing signalterminals 16 and modulation signal terminals 17 which may be selectivelyconnected to a signalV line 18 via a switch 19 having contact points19(a), 19(b), 19(c). .ffhehoscilloscope apparatus is preferablyseparated by a partition 21 from the room containing the subject for thepurpose of reducing the artifactual inuence of fields associated withthe generating apparatus and avoiding the possibility of any consciouseffort to affect correlation.

Disposed in a suitable position adjacent the cathoderay tube 12 is asupport such as table 22 upon which the subject may be positioned. Thesupport 22 may be any convenient arrangement which is comfortable forthe subject and which positions his eyes preferably directly in front ofthe screen 12 at a suitable distance, for example twelve inches. Astandard electroencephalograph (EEG) 23 is arranged with the sensingleads 24 thereofA preferably directly connected to the subjects scalp atthe points the physiological potential of which is tor be investigated.Inasmuch as' photic stimulation is to be investigated, the scalpconnections of primary interest will be those associated with the visualportion of lthe neurological system, such as, for example, the lowcentral to homolateral occipital connection.

They outputs of the EEG may be recorded by means of the individualchannel pens and a moving chart in a conventional manner by a recorder25. The output of a selected one of the EEG channels is supplied to ananalyzer 26 which may be an Offner automatic low frequency analyzer. TheOlner automatic low frequency analyzer receives the selected EEGpotential and by means of adjacent tuned band pass circuits separatesthe frequency components of the input wave from l1/2 cycles per second(C. P. S.) to C. P. S. These frequency components are individuallyrectified and accumulated during the EEG recording epoch so that therelative magnitude of the stored frequency components is available foroperating a suitable recorder. The recording in sequence of the 'storedfrequency components produces a frequency component curve for the EEGinput wave. A switch 27 is arranged for selecting one of the frequencycomponent outputs of the analyzer 26 as will be more fully describedhereinafter. Another switch 28 is arranged to connect the frequencycomponent from the switch`27 via contact 28(a) to the line 18 or viacontact 28(b) one of the channel outputs of the EEG may be connected tothe line 18. The line 18 may run directly to the switch 19 or mayinclude any desired operational circuit, for

example a time derivative circuit 29 selectively inserted by a switch31. The analyzed output of a selected channel may be supplied to therecorder 25 by a connection 32 as will appear hereinafter. If desired,timing markers of the periodicity of the quasi-visible rate sweep may berecorded by means of a signal via lead 30.

In Fig. 2 suitable arrangements are shown for operating an imageproducer 11 having means for producing a controlled image on the imagescreen 12. Included are a horizontal deflection force generator 33 and avertical defiection force generator 34, both supplied from a modulator35 with a 90 phase shift unit 36 interposed between the modulator andone of the generators 33, 34. The modulator 35 is supplied with asuper-visiblev rate sine wave carrier from an oscillator 36 andamplitude modulation signals therefor selectively by means of a switch37. The switch 37 provides at a contact 37 (a) external modulation ofany desired character and at contacts 37(b) and 37 (c) saw-toothmodulation of a rising or falling character as derived by phaseinversion in a phase inverter 38 of the saw-tooth wave generated by asaw-tooth generator 39. The generator 39 has a frequency adjustment 15and a synchronizing signal terminal 16. An elementary schematic diagramof a circuit Suitable for performing the oscillation generation andmixing functions of the system of Fig. 2 is shown in Fig. l of thepreviously referenced article. It will be understood that these circuitsare exemplary only and that many arrangements will be apparent to thoseskilled in the electronic and allied arts for producing the desiredVisual effects taught by the present invention. l

In Fig. 3 the viewing screen 12 has displayed thereon an expandingspiral 43-43 as the preferred visual pattern which may be generated bythe saw tooth modulated sine wave deflection forces 44, 45. The maximumdiameter of the outer spiral boundary 43 is made adjustable as well asthe inner boundary 43 which may be reduced to a point or substantiallyzero diameter. As has been explained, the spiral 43-43 is preferably sotightv that the visual pattern appears as an expanding circle. For thepurpose of illustration in Fig. 3 the sine waves 44, 45 and the spiral43-43' are shown on coarse scale.

Other expansion or contraction configurations for the 4light pattern maybe used in a particular examination. For example, if photic stimulationin response tolight fpatterns having diverse extensions or rates ofmotion with respect to two axes is desired, an elipse may be usedinstead of a circle. The figure need not be curved but may be square orrectangular as shown in Fig. 5 or some other regular shape which can bereadily generated. For certain controlled investigations a fragmentaryfigure may be employed. Thus in Fig. 4 by blanking the cathode-ray overa predetermined sector 46 or masking a portionrof the tube face 12, areaselective sector energizaltion of the retina may be obtained. Bycontrolling the `inner and outer diameters of a circular pattern, anydesired distribution of energy between the macular and peripheral areasof the retina is possible.

The diagnosis of a patient with the apparatus of the present inventionmay be made in accordance with the novel methods now to be described. Inan arrangement jsuch as shown in Fig. l a patient is placed in positioncomfortably to View the screen 12 of the tube 11 at a suitable distance,say l2 inches. The normal scalp-toscalp connections are made with theleads 24,to sense the physiological potentials which aregeneratedrbetween One signalpair is selected for analysis, for examplethe connections to the low central and homolateral occipital regionswhich provide response related'to the subjects visual system. Switch 19is positioned at contact 19(41), interrupting the feed-back circuitandmaking the expansion or contraction rate for the pattern controlledlas-to periodicity by means of the control 15. VThe amplifier channel ofthe EEG 23 to which the selected signal-pair of leads 24 supplies theinput signal has the output signal thereof applied to the analyzer 26.The output of the analyzer 26 produces the relative amplitudes of thefrequency components of the EEG potential which was applied to the inputof the analyzer 26. To record these frequency components they may beapplied to a pen of the recorder 25 via lead 32 in such a manner thatthe analyzed response is super-posed upon -the EEG potential recordingswhich have kbeen analyzed to produce the response. One suitablearrangement is provided by ten seconds (one epoch) recording of EEGpotentials with the analyzer response pen spatially displaced a distancealong the path of the record chart corresponding to a predetermined timeafter the epoch. This spacing of the pens is made to .correspond to thetime taken for the analyzer 26 to determine the frequency components ofthe signal within the epoch and the magnitudes of these components arethen recorded successively within 'the chart space of the epoch.

In order to establish a datum reference for each patient, at least oneset of data is recorded with the view of screen 12 obscured from thepatient. Preferably several sets of data are recorded with each patient,both for ,observing the variable pattern and with the screen 12obscured, and these sets will ordinarily be recorded in alternation.During the control epochs or sets the patient may run a maze with hiseyes by allowing his eyes to follow the pattern of the maze or stareinto the distance or at a blank card, in which event the alpha rhythmwill be recorded as for a normal subject; alternate epochs during whichthe pattern is observed produce definite peaks of EEG response at thepattern expansion or modulation frequency for normal subjects.

Representative data obtained by the instant method are shown in Figs. 6and 7. These figures are recordings of successive l second epochs duringwhich seven EEG potentials 51-57 were recorded. The recorded traces51-57 respectively are the EEG potentials existing between the followingscalp-to-scalp connections: 51 left-frontal to left central; 52left-central to left occipital; 53 right-frontal to right central; S4right-central to right occipital; 55 left occipital to right occipital;56 left-central to right-central; 57 left-frontal to right frontal.Marker pulses synchronized with the pattern expansion and rece'ived vialine 30 are recorded at trace 58. Superposed on the EEG traces of Fig. 6is an analyzer trace 59 giving the relative amplitudes of the frequencycomponents between 11/2 and 30 C. P. S. for the wave 54 while thesubject was running a maze. The succeeding epoch of Fig. 7 representsconditions identical with those of Fig. 6 except that the subject wasobserving an expanding circle having periodicity of ve per second. Theaccentuated response 6i) of the pattern frequency component of five C.P. S. is apparent.

The foregoing method provides by comparison technique informationrelative to the normal functioning of the neurological system associatedwith the subjects visual apparatus. The experimental conditions such asexpansion or contraction, frequency, phase, modulation waveform and thelike are all readily available for selection in wide variety by wellknown circuit techniques or equivalent apparatus. An examination over asubstantial range of any one of these variables is a time consumingprocess, however, during which the patient is likely to become fatiguedwith a concomitant loss of sensitivity of EEG response to photicstimulation. Furthermore, due to the vague nature of the relationbetween EEG potentials and the entire neurological system, conditionsmay change during an experimental procedure which would infiuence theconclusions obtained from the data.

The foregoing difficulties may be largely obviated and significantadditional data, further indicating characteristics of thevisual-neurological system, obtained by means of modified apparatus andmethods now to be described.

With these arrangements all of the techniques and advantages of closedloop systems become applicable to neurological diagnosis and studies.Closed-loop systems may be broadly classified as those systems having aninput control signal and an output response dependant in part on theinput control signal and in which the output response is utilized tomodify the effective input control signal. Closed loop systems whichinclude a human operator as a link in the feedback path are well knownand countless examples of this arrangement for control purposes areapparent every day. However, the use of a portion of the human system ina closed loop arrangement for determining involuntary characteristics ofthe human system has not heretofore been proposed.

In the arrangement of Fig. l the switch 19 may be moved to the contact19C and switch 28 may be connected to the contact 28(41). The switch 27,in selecting particular frequency components of an EEG wave, preferablyis provided the direct magnitude of the component as indicated in Fig.8. In Fig. 8 a frequency selection circuit 71 is indicated together witha rectifier 72 and a. storage capacitor 73 which normally accumulates acharge proportional lto the frequency component Selected by the circuit71 and stores this charge until the component is plotted, such as therelative magnitude of the individual frequency components 59 heretoforedescribed. For supplying an instantaneous magnitude of the frequencycomponent of the EEG potential selected by Vthe circuit 71 to the switch27, a switch 74 is provided to disconnect the rectifier 72 and capacitor73. The frequency component selected by the circuit 71 is thus oonnected directly to the output of the switch 27.

The voltage from the switch 27 is applied to the terminals 16 tosynchronize the expansion or contraction of the luminous pattern with orwithout being modified by the operator 29, as desired. For precisesynchronization the operator 29 may include clippino and shapingcircuits before differentiation to provide well defined synchronizingsignals.

For complete feedback control of the system of Fig. l the switch 19 maybe moved to the contact 19(1)). For this connection the switch 37 ,ofFig. 2 should be moved to the contact 37(41). With these connections andthe switch 28 at contact 28(a), the expansion or contraction modulationof the luminous pattern is directly under control of the frequencycomponent selected by the switch 27. For ease in transition betweenselected modulation and feedback controlled modulation it may bedesirable to provide a differential attenuator between the contacts 37(a) and 37( b). One form of such attenuatcr suitable for this purpose isindicated in Fig. 9. In Fig. 9 an arm 75 moves differentially over apair of potentiometers 76, 77 which have the signals from the terminals3701) and 37(1)) respectively impressed thereacross. The arm 75 suppliesthe input signal to the modulator. With this arrangement a response canbe initiated by manual control of the expansion frequency with thecontrol 15. At any Adesired point a selection of frequency component canbe made with the switch 2.7 and the control of the expansion graduallytransferred to full feedback control by shifting the arm 75.

In this specification and claims the light pattern generated by theperiodic predetermined motion of an elemental area of light shall bedefined as a scansion light pattern.

Obviously, many arrangements and procedures will be apparent to thoseskilled in the art in the light of the above teachings which areunderstood to be descriptive only and not in limitation. The inventiontherefore may be practiced in other than those manners hereinspecifically described without departing from the scope of the appendedclaims.

We claim:

l. The method of comparative photostimulation of a steps of recordingelectroencephalographic potentials of said organism while said sense issubject to a control condition, recording electroencephalographicpotentials of said organism while said sense is stimulated by apredetermined repetitive scansion light pattern, and comparing saidrecordings for the influence of said stimulation on said potentials.

2. Photo stimulation apparatus comprising, means for generating aquasi-visible rate scansion light pattern substantially in the form ofan expanding or contracting ring, means for varying said scansion rate,electroencephalographic means for sensing brain Wave potentialsinfluenced by said light pattern, means for developing modulationsignals in accordance with said potentials, means for differentiallyselecting control signals from said varying means and said modulationsignals, Vand means ,for controlling said scansion in accordance withsaid control signals.

3. Photostimulation apparatus comprising` oscillographic means forgenerating and exhibiting Ato the patient a quasi-visible rate scansionlight pattern substantially in the form of an expanding or contractingring, an electroencephalograph connected to saidA patient for sensingbrain wave potentials of the patient influenced by said light pattern, arecorder connected to said electroencephalograph for recording saidpotentials and connections from said oscillographic means to saidrecorder for supplying timing signals related to said scansion to saidrecorder to record timing marks whereby the record of said recordercorrelates the record of said brain wave potentials with the frequencyof said expanding or contracting ring indicated by said timing marks.

4. Photostimulation apparatus comprising a light pattern generatorhaving a viewing screen for displaying said pattern, means forcontrolling said generator to display a selectable quasi-visible ratescansion light pattern on said screen, an electroencephalograph forconnecting to a patient viewing said screen to detect brain wavepotentials influenced by said light pattern, a recorder connected tosaid electroencephalograph for recording said potentials, and means forcorrelating the record of said potentials with the frequency of saidquasi visible rate light pattern whereby the inuence of said frequencyon said patient can be determined.

5. Photostimulation apparatusA comprising a light pattern generatorhaving a viewing screen for displaying said pattern, means forcontrolling said generator to display a selectable quasi-visible ratescansion light pattern on said screen, an electroencephalograph forconnecting to a patient viewing said screen to detect brain wavepotentials influenced by said light pattern, and connections from saidelectroencephalograph to said generator for modifying said quasi visiblerate light pattern responsive to said potentials whereby the response insaid brain wave potentials to said photostimulation can be determined.

6. Photostirnulation apparatus comprising a light pattern generatorhaving a viewing screen for displaying said pattern, means forcontrolling said generator to display a selectable quasi-visible ratescansion light pattern on said screen, an electroencephalograph forconnecting to a patient viewing said screen to detect brain wavepotentials influenced by said light pattern, means for deriving signalshaving the fundamental frequency of said quasi visible rate from saidpotentials, and connections from said last named means to said generatorfor synchronizing the frequency of said scansion in response to saidsignals whereby the influence of said frequency on said patient can bedetermined.

7. Photostimulation apparatus comprising a light pattern generatorhaving a viewing screen for displaying said pattern, means forcontrolling said generator to display a selectable quasi-visible ratescansion light pattern on said screen, an electroencephalograph forconnecting to a patient viewing said screen to detect brain wavepotentials a influenced by said light pattern, means connected to saidelectroencephalograph'for generating modulation voltage signalsinaccordance with the wave formof said Ypotentials, and connections fromsaid modulation signal means :to said generator for controlling saidVscansion generator to vary said quasi visible rate pattern/in responseto said modulation voltage signals whereby the response in said brainwave potentials to said photostirnulation can be determined. Y

8. Apparatus according to claim 7 in Vwhich said means for developingmodulation signals includes ai'frequency selective device operable forselecting predetermined frequency components of said potentials.

- 9. The method of photostimulation of a living organism having a visualsense comprising the steps of exhibiting a predetermined repetitivescansion light pattern and stimulating said visual sense with said lightpattern, controlling a repetitive characteristic of said pattern to varyat a frequency below the threshold for persistance of vision, sensingbrain wavek potentials influenced by the frequency of said lightpattern, deriving signals having the fundamental frequency of saidrepetitive characteristic from said potentials, and synchronizing saidrepetitive characteristic of said pattern in response to said signalswhereby the influence of said frequency of said lightv pattern on saidpatient can be determined.

l0. The method of photostimulation of a living organism having a visualsense comprising the steps of exhibiting a predetermined repetitivescansion light pattern and stimulating said visual sense with said lightpattern, controlling a repetitive characteristic of said pattern to varyat a frequency below the threshold for persistance of vision, sensingbrain wave potentials influenced by the frequency of said light pattern,generating modulation voltage signals in accordance with a particularfrequency component of said potentials, and applying said signals tovary said repetitive characteristic of said pattern corresponding to thewaveform of said modulation voltage signals whereby the response in saidbrain wave potentials to said photostimulation can be determined.

1l. Photostimultion apparatus comprising a cathoderay oscilloscopehaving two pairs of deflection plates for deflecting the cathode-ray intwo perpendicular directions, a sine wave oscillator for generating avoltage having a freguency much higher than the threshhold forpersistence of vision, a voltage generator for generating a saw-toothvoltage wave form with a controllable range of fundamental freguenciesof substantially Zero cycles per second up to the order of thirty cyclesper second, a manually operable frequency selector for said saw toothgenerator, an amplitude modulator connected to said sine wave oscillatorand said saw-tooth generator for combining the voltages from saidoscillator and said generator to produce a modulated output voltage ofthe frequency of said sine Wave with amplitude modulation in accordancewith said saw-tooth waveform, connections for applying said modulatedoutput to one pair of said deflection plates, a ninety degree phaseshift circuit connected to said modulated output and the other pair ofsaid deflection plates whereby a visually continuons circle of light istraced on the screen of said oscilloscope and the diameter of saidcircle varies directly with the amplitude of said saw-tooth modulationwaveform and at the same controllable frequency, means for exhibitingthe varying diameter circle on said screen to a patient in a room freeof spurious electrical influences, an electroencephalograph connected tothe scalp of said patient, a

recorder for the brain wave potentials measured by saidelectroencephalograph, a timing mark pen on said recorder, a circuit foractuating said pen in timed relation with said saw-tooth modulation, anda frequency analyzer connected to said electroencephalograph fordetermining the magnitude of the particular frequency component of saidbrain wave potentials which are'induced by the frequency of varying saidvarying diameter circle whereby the response in said brain waves ofvarious frequencies corresponding to the selected Values of saidcontrollable frequency can be determined.

12. The method of photostimulation of a living organism having a visualsense comprising the steps of exhibiting a predetermined repetitive10W-level light pattern and stimulating said visual sense With saidpattern, controlling a repetitive characteristic of said pattern to varyat a frequency below the threshold for persistance of vision, and

indicating the iniluence of said stimulation with an in- 10 ducedelectrophysiological potential of said organism containing frequencycomponents corresponding to said frequency of said light pattern.

References Cited in the Ile of this patent UNITED STATES PATENTS Oct. 8,1946 Aug. 1l, 1953 Dec. 22, 1953 March 14, 1942, pages 884-5. Library.)

(Copy in Scientific

